Commodity fumigation process and apparatus

ABSTRACT

Processes and devices for producing a heated non-flammable gaseous fumigant for commodity fumigation use by application of a carrier gas, such as carbon dioxide (102), to a heater (103) either prior or subsequent to mixing the gas with a toxic agent gas, such as methyl bromide (213), such as by means of a mixer (214) and applying the mixture of gasses as a commodity fumigant to eradicate target peats infesting a commodity stored within an enclosed volume (106). In the preferred embodiment, the gaseous carbon dioxide is formed by flashing liquid carbon dioxide (614) directly to the gaseous state in a heater (630) prior to being mixed with methyl bromide (654) in a mixer (648) and applying the mixture of gases as a fumigant to a commodity held within an enclosure (668).

TECHNICAL FIELD

This invention relates generally to commodity fumigation processesutilizing substantially reduced concentrations of toxic agent so as tobe less hazardous during application and result in substantially lesshazardous quantities of residue at the completion of the process, andmore particularly, to the use of carbon dioxide as an agent to carry,disperse and increase the effect of a toxic agent such as methyl bromideto be used to eradicate pests located in the commodity or othersubstance to be fumigated.

BACKGROUND ART

Conventional commodity fumigation processes have required highconcentrations of a toxic agent such as methyl bromide in order toachieve effective levels of eradication of a target pest infesting thebulk-stored commodity. If the toxic agent is methyl bromide, the presentpractice of using at least three pounds of the agent per one thousandcubic feet of commodity volume in order to achieve an effectiveconcentration of the methyl bromide in the commodity. However, by reasonof currently effective United States statutes and treaties, the quantityof methyl bromide used in fumigants to be applied to commodities must bereduced by one-half by the year 2000. For applications such as thefumigation of commodities, such as grains, for example, there ispresently no equally effective substitute for methyl bromide in use. Theconventional fumigation processes for commodities by the use of methylbromide do not provide for satisfactory kill rates for the target pestsat the reduced methyl bromide concentrations to be required forenvironmental reasons.

In this respect, tests have found that use of methyl bromide at suchconventional concentrations in commodities has the potential to resultin toxic residues which remain at a dangerous level after the process iscompleted. This level of residual toxic agent constitutes anunacceptable risk to workers handling the fumigated commodity after theprocess has been completed.

Toxic agents other than methyl bromide have been used in commodityfumigation processes. The levels of certain other toxic agents requiredto achieve the similar toxicity effects also result in currentlyunacceptable levels of toxic residue, requiring long aeration timesafter the process has been completed.

Typical attempts to solve the problem of residual toxicity incommodities have attempted to use carbon dioxide to control insects instored grain. This method, set forth in U.S. Department of Agriculturebulletin AAT-S-13/April 1980, requires a carbon dioxide concentration ofabout 60% to achieve 95% control of most stored grain insects after afour day exposure at temperatures of twenty-seven degrees Celsius orhigher. An alternate method of using a low oxygen-nitrogen atmospheremust be held for ten or more days.

U.S. Pat. No. 4,989,363 discloses a process for protecting storedcommodities in substantially a gas-filled enclosure of a pesticidalatmosphere comprising 6-100% carbon dioxide.

U.S. Pat. Nos. 4,651,263 and 4,756,117 have attempted to solve theproblem. They teach that bulk commodities such as grain infested bypests may be fumigated with phosphine gas. However, at lowconcentrations, the gas must be maintained three to four weeks. The timemay be shortened if the atmosphere is enriched with up to 30% carbondioxide. The temperature of this process is critical as phosphine gas isexplosive. Therefore, raising the temperature of the fumigant gas todecrease the concentration of the fumigant gas by weight cannot be usedsafely in that process.

Thus there has long been a need for a commodity fumigation process whichmay utilize toxic agents at lower levels of concentration than presentlyused in the art while providing a completed fumigation process in ashorter time than has been possible heretofore.

It is desired that at the end of this process the residual levels oftoxic agent be significantly reduced to lower, safer toxic limits sothat the commodity treated may be aerated and available for use earlierthan heretofore practical without unacceptable residual levels of toxicagents which would otherwise result in an unacceptable health hazard.

It is further desired that the process itself be environmentally saferthan those process presently used in the art. It is further desired thatthe gasses used be non-volatile so that they may be heated to allow thegasses to expand to penetrate the commodity mass being treated, therebyreducing the weight of fumigant acquired by the process to be ultimatelyreleased into the atmosphere.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide animproved commodity fumigation process which reduces the amount of toxicagent by at least a factor of from about one-half to about one-tenthwhile not extending the current exposure times.

It is another object of this invention to provide a commodity fumigationprocess with a shorter treatment time along with reduced toxic residualsin commodities.

It is another object to achieve reduced potential toxic residue in thecommodity from the fumigation process to levels recommended byenvironmental agencies.

It is yet another object to provide a commodity fumigation process whichdoes not pose a danger from auto-ignition of the gasses used.

It is yet another object to provide a commodity fumigation process whichsignificantly reduces the environmental impact of the process ascompared to those presently used in the art.

The above and other objects of the present invention are achieved,according to a preferred embodiment thereof, by providing an improvedcommodity fumigation process of using a non-flammable liquid cryogenicmaterial which is vaporized to function as a carrier for the toxicagent. Carbon dioxide is presently preferred, although nitrogen or othernon-flammable cryogenic liquids may be utilized to advantage withrespect to conventional practice. The utilization of carbon dioxide gasas a carrier has been found to effect the desired kill rate with lowerlevels of toxic agent. Concentrations by weight of carbon dioxide andtoxic agent are further reduced by including the step of heating thecarbon dioxide gas before releasing the carbon dioxide into the volumeof the mass of the commodity to be treated. The heating expands thecarbon dioxide and, therefore, the toxic agent when mixed with thecarbon dioxide to provide a driving force for the fumigant, providingeffective fumigation of the commodity with less gas and shorter time ofapplication periods.

In a preferred embodiment, the application of a gas mixture utilizing atoxic agent such as methyl bromide with carbon dioxide in the gaseousstate to the commodity mass allows the quantity by weight of methylbromide to be reduced substantially from the amount otherwise requiredin conventional fumigation treatments. Concentrations by weight of lessthan fifteen percent of carbon dioxide gas to air in the volumecontaining the commodity being treated allow effective fumigationresults to be achieved.

While the carbon dioxide may be mixed with a toxic agent and injectedinto a gas cylinder to be utilized at the fumigation site, in thepreferred embodiment an apparatus including a container of liquid carbondioxide at a relatively low pressure and a toxic gas cylinder at an evenlower pressure with appropriate controls, a heater for flashing liquidcarbon dioxide directly into gaseous carbon dioxide, and a mixingchamber for mixing heated carbon dioxide with the toxic agent areutilized to provide a heated gaseous fumigant. The liquid carbon dioxideis heated as it passes through the heater to "flash" directly into thegaseous state as its pressure decreases, rather than passing into thesolid state and subsequently evaporating, as has been the practiceheretofore. The heated gaseous carbon dioxide is then mixed with thetoxic agent, as for example by aspirating the toxic agent into thestream of heated gaseous carbon dioxide by means of a mixer disposedbetween the heater and the fumigation site. Controls are provided in theliquid carbon dioxide flow line and the toxic agent flow line to insurethat the reduced concentration of the toxic agent in the fumigantgaseous mixture is achieved.

The heated gaseous carbon dioxide used in the process causes thecombined gaseous mixture of carbon dioxide and toxic agent, that is, thegaseous fumigant, to continue to expand after entering the commoditymass to be fumigated, thereby further reducing the quantity of toxicagent required and assisting in dispersing the fumigant to eradicate thetarget pest by driving the toxic agent into the mass to be fumigated,rather than relying on the greater weight of the fumigant with respectto air to displace air in the fumigation process, as has been thepractice heretofore.

BRIEF DESCRIPTION OF DRAWINGS

The above and other embodiments of the present invention may be morefully understood from the following detailed description, taken togetherwith the accompanying drawing, wherein similar reference charactersrefer to similar elements throughout, and in which:

FIG. 1 is a schematic view of the apparatus of the present inventionshowing the flow of fumigant gas derived from sources of gaseous carbondioxide and a toxic agent held in conventional high pressure cylinders;

FIG. 2 is a schematic view which represents a control circuit for theapparatus of FIG. 1;

FIG. 3 is a perspective view of an embodiment of the invention for usewith the embodiments illustrated in FIG. 1 and FIG. 2;

FIG. 4 is a rear elevational view of the device shown in FIG. 3;

FIG. 5 is a schematic drawing of another embodiment of the invention asillustrated in FIG. 1 for use with the embodiment of FIG. 3; and

FIG. 6 is a schematic view of the presently preferred embodiment of thepresent invention utilizing liquid carbon dioxide contained at arelatively low pressure and temperature.

MODES FOR CARRYING OUT THE INVENTION

Fumigation of a commodity mass in accordance with the method of FIGS. 1and 2 may be performed by injecting a gaseous mixture of predeterminedamounts of carbon dioxide and a preselected toxic agent into an enclosedvolume containing the commodity. Some commodities may already be storedin an enclosed volume such as a shipping vessel, warehouse or other typeof chamber. If not, the commodity is moved into such a contained volume,which is sealed against excessive leakage in any acceptable conventionalmanner, such as by use of a tarpaulin, if necessary.

A cylinder of compressed gaseous carbon dioxide at room temperature hasinjected therein a toxic agent such as methyl bromide, in the range of 4ounces to 2 pounds per 22 pounds of carbon dioxide. The mixture ofcarbon dioxide and toxic agent is injected into the enclosed volume inthe range of from as little as 8 pounds per 1000 cubic feet of aircontained in the commodity containing volume, (approximately a 7% carbondioxide concentration in the air within the volume) to as much as 22pounds per 1000 cubic feet of air contained in the commodity containingvolume (approximately a 20% carbon dioxide concentration in the air).However, carbon dioxide gas at an initial 10% concentration with toxicagent concentrations as low as 2% of the fumigant gas mixture achieveacceptable fumigation results.

The preferred mixture of 8 ounces of toxic agent with 11 pounds ofcarbon dioxide injected per 1000 cubic feet of air in the enclosedvolume results in a toxic agent concentration of 4.35% in the carbondioxide mixture, with a carbon dioxide concentration of about 10% in thefumigant--air mixture.

It has been found that by passing the fumigant through a heat exchangerbefore being injected into the enclosed volume for mixing with the aircontained therein, the fumigation results are enhanced. Apparently theheating of the gaseous fumigant produces two improved results. The firstappears to be to reduce the amount of the fumigant required to treat theenclosed volume because heating causes the fumigant to expand. Second,because carbon dioxide is heavier than air, it may tend to accumulate inlow areas. The increased temperature of the fumigant causes the carbondioxide to rise and mix with the air instead of layering at the base ofthe enclosed volume. As the vapor pressure of carbon dioxide and thetoxic agent are different, the heating also encourages the mixture ofthe gasses forming the fumigant to stay together, thus achieving thedesired synergistic effect of increased respiration rate in the targetpest due to the increased carbon dioxide concentration while the toxicelement of the mixture being respirated maintains a toxic effect uponthe target pest. Of course, fans may be used to assure continuedcirculation of the fumigant within the enclosed volume after initialinjection, as the heated mixture cools.

Other toxic agents may be mixed with the carbon dioxide. The toxicagents commonly used for commodity fumigation are methyl bromide andsulfur dioxide. Methyl bromide has been the toxic agent of choice in thefumigation industry but is currently under consideration byenvironmental and occupational safety agencies for withdrawal from themarket due to adverse reactions to concentrations of residues whichaccumulate in treated commodities. However, by virtue of the reducedconcentrations of methyl bromide resulting from the practice of thepresent invention, methyl bromide again is an acceptable component of afumigant with respect to safety and environmental considerations.

Referring now to FIG. 1, a fumigation apparatus, generally designated as100, for the practice of the invention is shown as utilized in afumigation process. An enclosed space 106 containing the commodity to betreated is sealed under a tarpaulin 104. The tarpaulin 104 isconventional and may be of plastic or rubber impregnated material toform an essentially air tight barrier, thereby enclosing the space 106to be treated.

A high pressure (1,700 pounds per square inch and above) gas cylinder102 having a pressure gauge and control valve mounted on it andcontaining the mixture of carbon dioxide and toxic gas therein atpreselected proportions is mounted on a scale 101. The weight of thecombined cylinder 102 and gas mixture therein is noted. An input hose109 is connected the output control valve of the gas cylinder 102 andthe input of a heat exchanger 103. A manifold or engine coolant heatexchanger commonly used in the fumigation industry may be used. The heatexchanger may raise the temperature of the fumigation gas to the rangeof from 70° F. to 250° F. An output hose 110 connects the output of theheat exchanger 103 to an outlet 111 mounted within the tarpaulin 104. Aflow meter 105 and temperature monitor 108 may be incorporated in theoutput hose 110 as safety devices to assure the flow and temperature ofthe fumigant are within acceptable ranges. The control valve of the gascylinder 102 is opened and the scale 101 is monitored while theprecalculated weight of fumigant gas is released into the enclosed space106. A plurality of fans 107 may be placed within the enclosed space 106to assist in dispersal of the fumigant gas.

FIG. 2 is another embodiment of the device, generally designated 200.Instead of mixing the carbon dioxide and toxic agent in a single tank,the carbon dioxide may be the only gas in one gas cylinder 102 and thetoxic agent may be contained in another gas cylinder 213. Controlledflow meters 105 and 212 may be mounted at the output of each gascylinder 102 and 213 to control the flow of each gas through a manifold214 and the input hose 109 to the heat exchanger 103. The output hose110 may contain the temperature monitor 108 and conducts the heated mixof carbon dioxide and toxic agent through the outlet 111 into theenclosed space 106 as above. The flow meters 105 and 212 may be set toprovide the mix of gasses recommended to achieve the fumigation effectdesired.

For general fumigation processes, especially for large scaleapplication, or numerous applications during a day by a qualifiedfumigation person, a skid mounted embodiment may be provided. FIG. 3 isyet another embodiment of the invention, generally designated 300, whichmay be mounted on a skid and lifted onto a truck for transportation tothe job site.

A plurality of gas cylinders 302 may be mounted within a closedcontainer 320 having skids 321 on the bottom surface. The gas cylinders302 may contain a mix of carbon dioxide and toxic agent as described forembodiment 100 above or each gas cylinder 302 may contain only one gasas described for embodiment 200 above. The number of gas cylinders 302containing only one gas may be preselected to allow the mixing of carbondioxide and toxic agent in the recommended proportions withoutexhausting one type of gas.

A control panel 319, see FIG. 4, may be incorporated as one side of theclosed container 320. The control panel may contain the connector forthe input hose 110 which conducts the gas mixture from embodiment 300 tothe heat exchanger 103 as described in embodiments 100 and 200.

The art of controlling gas flow, mixing gasses, and presetting theamount of gas to be mixed and discharged from the gas cylinders 302within the closed container 320 is well known in the art. The controlpanel may contain a flow meter 105 for the carbon dioxide, a flow meter212 for the toxic agent, and a flow controller 318 for setting theamount of mixed carbon dioxide and toxic agent to be discharged fromembodiment 300.

Appropriate safety latches may be incorporated into the connector 317 ofinput hose 110 to prevent the discharge of gas from embodiment 300unless the input hose 110 is attached.

FIG. 5 is an embodiment, generally designated 500, which may beincorporated into embodiment 300 of FIG. 3. A plurality of supplycylinders 502 are each connected to a controllable valve 503. A valvecontroller 504 is present to produce the preselected mix of gassescontained in the cylinders 502. The controllable valve 503 is connectedto a mixer 505. The mixer 505 may be attached to the safety latchconnector 317 to which the input hose 110 is attached.

Embodiments 100, 200 and 300 may be used to transport the fumigants tobe applied to treat commodities and food stuffs in any appropriateenclosed area or chamber, such as a vehicle, silo, etc.

The tarpaulin 104 may be used to enclose a volume of soil rather than acommodity-containing space 106. The embodiments 100, 200 and 300 may beused as the source of fumigation gases to treat the soil for targetpests such as phylloxera.

While it is possible to apply the methyl bromide or other toxic agent bymixing the toxic agent directly with carbon dioxide under pressure in ahigh pressure cylinder of the type conventionally used to store carbondioxide at a pressure of up to about 1700 psi, such premixing has aserious disadvantage if the entire cylinder contents are not used in asingle fumigation application. Because of the differing physicalcharacteristics of the toxic agent and carbon dioxide, the concentrationof toxic agent with respect to carbon dioxide will change as thecylinder is being emptied. Consequently, for many applications, theadvantages of the present invention may be lost by such premixing. Inaddition, the capacity of such high pressure cylinders is relativelylimited, so that the quantity of such cylinders which must betransported to the fumigation site may be relatively large.

High pressure carbon dioxide cylinders may dispense carbon dioxideeither as a liquid or as a gas, depending upon the particular deliverysystem embodied in the cylinder. If liquid carbon dioxide is dispensed,the pressure at which the carbon dioxide leaves the high pressurecylinder is so great as to make the use of the liquid carbon dioxideimpractical for the present invention. The pressure cannot be reducedthrough the use of a pressure regulator, as the addition of a pressureregulator to the flow of liquid carbon dioxide from a high pressurecylinder is prohibited by current safety standards.

If gaseous carbon dioxide is dispensed from the cylinder, the use of apressure regulator to reduce the pressure of the flow is required.However, the pressure regulator with either reduce the flow quantity tosuch a low value as to greatly prolong the fumigation process andpreclude the realization of many of the advantages of present invention,or, with an increase in flow, the gaseous carbon dioxide may solidify assnow, making it impractical for use in the present invention.

Because of the above deficiencies in the use of high pressure cylindersas the source of the gaseous carbon dioxide mixed with the toxic agentto form the fumigant in the present invention, the presently preferredembodiment of the invention, both in its apparatus and in its methodaspects, utilizes liquid carbon dioxide stored in insulated containers,typically of the Dewar flask type, at an operating pressure currentlytypically between 100 and 350 psi, although pressures up to 500 psi canbe used if the currently available storage reservoirs are modifiedaccordingly. Maximum pressure is controlled by a conventional safetyvalve which vents gaseous carbon dioxide formed by evaporation of liquidcarbon dioxide as it warms.

Referring now to FIG. 6, there is shown a schematic diagram of thepresently preferred apparatus for performing the presently preferredmethod of the present invention. In FIG. 6, a fumigation system 610 hasa Dewar or mini-bulk type storage vessel 612 within which is storedliquid carbon dioxide 614. It will be understood that the space withinthe storage vessel 612 above the liquid carbon dioxide 614 is filledwith gaseous carbon dioxide. A gaseous carbon dioxide outlet pipe 616connects the interior of the storage vessel 612 containing in thegaseous carbon dioxide to a safety valve 618 whose outlet 620 is ventedto the atmosphere. The purpose of the safety valve 618 is to avoid anexcess in pressure within the storage vessel 612 resulting from thecontinuous evaporation of the liquid carbon dioxide 614 into gaseouscarbon dioxide, since the storage vessel 612 is not, in this embodiment,provided with an external source of coolant. However, the presentinvention is not limited to a storage vessel 612 which is not providedwith external sources of coolant. Even in such an event, the safetyvalve 618 would be required in order to avoid the potential foroverpressure within storage vessel 612 in the event of failure of thecooling system.

A liquid carbon dioxide outlet line 622 extends from within the interiorof the storage vessel 612 at the lower portion thereof to a controlvalve 624, which is conventional in form. The control valve may beeither manually or automatically operated, depending upon the particularcontrol system utilized with the apparatus 610. A liquid carbon dioxideflow meter 626 is connected between the control valve 624 and an inlet628 to a heater 630 by means of conduits 632, 634, respectively. Whenthe control valve 624 is open, liquid carbon dioxide flows from thestorage vessel 612 through the control valve 624, the flow meter 626,and a heater inlet 628 into a heater 630. The quantity of liquid carbondioxide flowing through the flow meter 626 is recorded by the flow meter626, which preferably is of the positive displacement type. Such flowmeters are commercially available and, in the preferred embodimentdescribed with respect to FIG. 6, should have the capability ofrecording the quantity of flow of liquid carbon dioxide at flow rate offrom one to ten gallons per minute.

The heater 630 may use gas, oil, or an electrical source of heat,although for portable embodiments of the invention, a liquid petroleumgas fuel source is preferred. A container 635 contains liquid petroleumgas, and is connected to the heater 630 through a control valve 636 andassociated gas lines 637, 638. A heater vent line 640 exhausts theproducts of the combustion of the liquid petroleum gas to theatmosphere. In the preferred embodiment, the heater 630 is a highpressure heater, which may be, for example, designed to heat water. Forsafety purposes, the heater should be capable of withstanding pressuresin excess of 1500 psi. The primary function of the heater 630 is toconvert the liquid carbon dioxide applied to the heater inlet 628 intoheated gaseous carbon dioxide by "flashing" the liquid phase carbondioxide directly into the gaseous phase, without passing through thesolid phase. Consequently, the heater 630 must have a sufficiently highheat output to maintain the temperature of the carbon dioxide as itpasses from the liquid to the gaseous phase sufficiently high to avoid apressure drop sufficient to cause the liquid carbon dioxide to beconverted to the solid phase. Therefore, the heater 630 should convertthe liquid carbon dioxide directly into gaseous carbon dioxide toprovide a gaseous carbon dioxide flow having a temperature preferably ofat least 120° F., and above about 70° F. at a minimum as it is appliedto the commodity-holding space, to provide for satisfactory expansion.

The heater 630 has an outlet 642 to which an outlet line 644 isconnected at one end. The temperature of the flow at the heater outlet642 should exceed ambient temperature under normal operating conditions.The second end of the outlet line 644 is connected to an inlet 646 of amixer 648. Disposed in the outlet line 644 between the heater outlet 642and the mixer inlet 646 are a temperature gauge 650 and a pressure gauge652, for use in monitoring the temperature and pressure of the gaseouscarbon dioxide output of the heater 630. The control valves 624 and 636are utilized to ensure that the temperature of the gaseous carbondioxide output of the heater is within the desired parameters. In orderto ensure that the liquid carbon dioxide is flashed to gaseous carbondioxide and provide a sufficient flow rate of the gaseous carbon dioxidefor fumigation purposes, typically the heated gaseous carbon dioxidemonitored by the meters 650, 652 will be within a temperature range ofbetween 70° F. and 250° F. and at a pressure less than the pressurewithin the storage vessel 612.

A toxic agent container 654 contains a toxic agent, methyl bromide inthe presently preferred embodiment, which is to be mixed with the flowof heated gaseous carbon dioxide. Typically, the toxic agent will beunder pressure, although normally at a comparatively low pressure, suchas 150 psi, as compared to the pressure within the storage vessel 612,which typically is between 150 psi and 350 psi. Of course, the upperlimit of the pressure within the storage valve is controlled by thesetting of the safety valve 618. Pressures as high as 500 psi may beutilized, if desired, although the presently preferred maximum pressureis 350 psi.

The mixer 648 has a toxic agent inlet 656 which is connected to thetoxic agent containers 654 through a toxic agent inlet line 658, whichincludes a toxic agent flow control valve 660 and toxic agent flow meter662. The toxic agent flow meter 662 is preferably of the positivedisplacement type, so as to record the total quantity of toxic agentwhich is applied to the toxic agent inlet 656. The mixer 648 has a mixeroutlet 664 to which a fumigant outlet line 666 is connected. Thefumigant outlet line 666 is an open conduit which extends into anenclosed space 668, which contains a commodity (not shown) which is tobe fumigated for pests.

A tarpaulin or its equivalent 670, if necessary, further encloses thespace 668 so as to prevent the passage of air from outside the tarpaulin670 into the space 668. In the particular application diagrammaticallyillustrated in FIG. 6, the tarpaulin 670 is held on a supporting surface(not shown) by weights 672 in conventional fashion. As isdiagrammatically illustrated, the tarpaulin 670 has ballooned away fromthe space 668, by reason of the pressure of the carbon dioxidecontaining fumigant which has passed through the mixer 648 and thefumigant outlet line 666 into the interior of the space 668. The space668 has inherent air leakage, so that as the carbon dioxide containingfumigant mixes with the air in the space 668, so as to displace aportion of the air from the space 668 through leakage in the tarpaulin670, the displaced air passes into the atmosphere. However, thetarpaulin is sufficiently gas-tight so that it balloons out from theincreased pressure within the space resulting from the application ofthe fumigant mixture containing gaseous carbon dioxide.

As was noted above, in the process aspects of the preferred embodiment,gaseous carbon dioxide which has passed from the heater outlet 642 andis applied to the mixer inlet 646 is at a higher pressure than thepressure of the toxic agent within the container 654. In the preferredembodiment, the toxic agent within the container 654 is mixed with theflow of gaseous carbon dioxide in the mixer so that the mixture ofgaseous carbon dioxide and toxic agent is applied to the structure 668.In order to ensure efficient mixing of a sufficient quantity of toxicagent with the gaseous carbon dioxide, in the preferred embodiment thepressure of the carbon dioxide flowing through the mixer 648 istemporarily reduced during its transit through the mixer 648 by the useof a Venturi or similar arrangement. The toxic agent inlet 656 islocated downstream from the carbon dioxide inlet 646 at a point where,by reason of the pressure reduction accomplished temporarily in the flowof the gaseous carbon dioxide through the mixer 648, with, of course, anincrease in velocity of gaseous carbon dioxide, the pressure of thetoxic agent is in excess of the pressure of the gaseous carbon dioxideflow, so that the toxic agent flows into the stream of gaseous carbondioxide. While in the preferred embodiment, the reduction in pressure ofthe carbon dioxide flow is only temporary, the mixer 648 can beconfigured, of course, to provide a permanent reduction in the pressureof the carbon dioxide flow, although such a permanent reduction is notpresently preferred, when a tarpaulin or other temporary sealing elementis used to seal the space 668, by reason of the potential loss offumigation efficiency resulting from loss of the "ballooning" effect ofthe tarpaulin 670.

Normally, the rate of withdrawal of liquid carbon dioxide from thestorage vessel 612, which results in a decrease in the pressure of thegaseous carbon dioxide in the storage vessel 612, is compensated for bythe continuing increase in pressure of gaseous carbon dioxide resultingfrom evaporation of the liquid carbon dioxide by reason of absorption ofheat from outside the storage vessel 612. By reason of the size of thestorage vessel 612, or other factors, the withdrawal of the liquidcarbon dioxide could result in a sufficiently large decrease in pressureof the gaseous carbon dioxide to approach the pressure which, for thetemperature of the liquid carbon dioxide, the liquid carbon dioxide mayconvert into solid carbon dioxide. To avoid this conversion, a highpressure cylinder 673, filled with carbon dioxide, typically at apressure in excess of 1500 psi, is connected to a storage vessel gasinlet 674 by a high pressure gas line 676. A control valve 678 controlsthe flow of high pressure gas from the cylinder 673 to the inlet 674.The control valve 678 may be either manually operated, or automaticallyoperated at such time as the pressure of the gaseous carbon dioxidewithin the storage vessel 612 drops below a pre-selected pressure, toincrease the pressure within the storage vessel 612 to avoid conversionto solid carbon dioxide.

Typically, in the operation of the apparatus 610 shown in FIG. 6, theratios of carbon dioxide and toxic agent for the volume of air in thespace containing the commodity described heretofore with respect to theembodiments of FIGS. 1 and 2 are utilized, to achieve a carbon dioxideconcentration of about 10% in the fumigant-air mixture containing thecommodity to be fumigated, and a flow rate of from one to ten gallonsper minute of liquid carbon dioxide is utilized. The total weight ofcarbon dioxide required by the volume of air contained in the spaceholding the commodity to be fumigated is determined, the presentlypreferred amount being eleven pounds per thousand cubic feet of air toprovide the 10% concentration of carbon dioxide. The flow of carbondioxide through the control valve 624 is terminated after the flow meter626 indicates that the desired quantity of carbon dioxide has beenapplied to the heater 630. During the process of the flashing of theliquid carbon dioxide in the heater 630, the toxic agent control valve660 is opened, and toxic agent from the toxic agent storage cylinder 654flows into the mixer inlet 656 through the toxic agent flow meter 662.At such time as the desired amount of toxic agent has passed through thetoxic agent flow meter 662, the toxic agent flow control valve 660 isclosed, terminating the flow of the toxic agent. The flow of carbondioxide is then terminated either contemporaneously or at suchsubsequent time as the desired amount of carbon dioxide has been flashedand flowed into the space 668. While it is possible to flow the toxicagent into the space 668 and then commence the flow of carbon dioxide tocomplete the fumigant mixture, such a practice is not presentlypreferred in the process aspects of the invention.

By the practice of the present invention, the efficient eradication ofthe target pest can be accomplished even though the weight of toxicagent required is reduced by at least one-half and, in certaininstances, seven-eighths or more. In the practice of the presentinvention, the presently preferred dosage range by weight of toxic agentfor commodity fumigation is from one-third to one-sixth the weight oftoxic agent conventionally used. The practice of the present inventionthus permits the weight of toxic agent required for eradication of thetarget pest to be reduced to less than one-half of that which wasrequired to be used heretofore, while still effecting the requirederadication of the target pest.

The use of the heated carbon dioxide increases the toxic effect providedby the toxic agent over and above the toxicity provided by either carbondioxide or the toxic agent alone. For example, the combination of heatedcarbon dioxide and methyl bromide produces a synergistic effect as totoxicity, and as to penetration of the fumigant as well, while greatlyreducing the quantity of toxic agent required and thus greatly reducingthe health risk to those involved in the fumigation operation or thesubsequent handling or ingestion of the fumigated commodity.

The term "commodity" as used herein with reference to fumigationprocesses, refer to fumigation of an enclosed volume, the major portionof which contains solid materials, grains or nuts, for example, asdistinguished from a structural fumigation process, in which the majorportion of the volume being fumigated consists of air, although thefumigation of solid materials may be inherently accomplished in thestructural fumigation process, such as fumigation of grain beingtemporarily stored prior to use in a flour mill being fumigated or flourbeing temporarily stored prior to use in a bakery being fumigated, asdistinguished from the fumigation of a grain-filled storage silo, forexample.

The invention claimed is:
 1. In a process for fumigating a commoditycontained within an enclosed space, the steps of:providing a reservoirof a non-flammable cryogenic liquid; continuously maintaining saidcryogenic liquid at a pressure of less than 500 pounds per square inch;providing a flow of cryogenic liquid from said reservoir; heating saidflow of cryogenic liquid to a temperature such that the cryogenic liquidflashes directly into its gaseous state to provide a flow of heatednon-flammable gas; applying the flowing mixture of heated gas to thespace to be fumigated; providing a source of a toxic agent; providing aflow of said toxic agent from said source; and mixing the toxic agentwith the flow of heated gas to produce a fumigant by introducing thetoxic agent into the flow of heated gas before the heated gas enters thespace.
 2. The process of claim 1, and in which the flow of toxic agentis at a first pressure which is less than the pressure of the flow ofthe heated gas, and including the step ofreducing the pressure of theflow of heated gas to a second pressure which is less than said firstpressure prior to mixing said toxic agent with said heated gas flow. 3.The process of claim 2, and in which the cryogenic liquid is liquidcarbon dioxide.
 4. The process of claim 3, and in which the flow ofgaseous carbon dioxide is at a pressure in excess of 100 psi prior tobeing mixed with the toxic agent.
 5. The process in any one of claims 3or 4, and in which the toxic agent is methyl bromide and the fumigantwithin the space is comprised of a mixture of methyl bromide and carbondioxide in a ratio of from 4 ounces to 2 pounds of methyl bromide per1000 cubic feet of space volume and at least eight pounds of carbondioxide per 1000 cubic feet of space volume.
 6. The process of claim 5,and in which the reservoir of liquid carbon dioxide is maintained at apressure in excess of 100 pounds per square inch, and in which thetemperature of the gaseous carbon dioxide when applied to the space isin excess of 70° F.
 7. The process of claim 6, and which the liquidcarbon dioxide is maintained at a pressure of between 150 and 300 poundsper square inch for substantially all of the period during which theliquid carbon dioxide is being flashed to gaseous carbon dioxide, andwhich the temperature of the carbon dioxide after flashing is in excessof 120° F.
 8. Apparatus for producing a gaseous fumigant for use as acommodity or soil fumigant comprising:first storage means for storingcarbon dioxide in a liquid phase at a first pressure which is alwaysless than 500 pounds per square inch; second storage means for storing atoxic agent at a second pressure; a heater having an inlet and an outletand operable to flash liquid phase carbon dioxide applied to said inletdirectly into gaseous carbon dioxide; fluid passage means connectedbetween said first storage means and said heater and selectivelyoperable to apply a selectable quantity of said liquid carbon dioxide tothe heater inlet as a liquid; mixing means having an outlet, a firstmixer inlet for receiving the heated gaseous carbon dioxide from saidheater and a second mixer inlet located downstream from said first inletfor receiving toxic agent from said second storage means, said mixingmeans being operable to mix said heated gaseous carbon dioxide from saidheater and said toxic agent when applied to said second inlet to form aheated gaseous mixture thereof and to apply said heated gaseous mixtureto said mixing means outlet; second fluid transfer means comprised byfluid conduit open along its length and operable to transfer the gaseouscarbon dioxide passing from said heater outlet to said mixing meansfirst inlet; third fluid transfer means selectively operable to transfera selectable quantity of said toxic agent from said second storage meansto said mixing means second inlet; and fumigant transfer meanscomprising a fluid conduit open along its length and having an inletconnected to said mixing means outlet and an outlet remote therefrom andoperable to transfer the heated gaseous mixture passing from said mixingmeans outlet to the fumigant transfer means outlet.
 9. Apparatusaccording to claim 8, and includingmeans selectively operable to addgaseous carbon dioxide to said storage means to reduce a pressure dropwithin said storage means resulting from withdrawal therefrom of liquidcarbon dioxide.
 10. Apparatus according to claim 8, and in which theheater is operable to flash the liquid carbon dioxide into gaseouscarbon dioxide which leaves the heater outlet at a temperature in excessof 120° F.
 11. Apparatus according to either claim 9 or claim 10, and inwhich the heater is operable to flash the liquid carbon dioxide intogaseous carbon dioxide which leaves the heater outlet of a pressure inexcess of 100 pounds per square inch.
 12. Apparatus according to claim11 and in which the second pressure is less than the pressure of theheated gaseous carbon dioxide as it leaves the heater outlet, and inwhich said mixing means includes means operable to reduce the pressureof the heated gaseous carbon dioxide as it is being mixed with the toxicagent to a pressure less than said second pressure.